The present invention relates to a diaphragm fluid pump, and more particularly to a diaphragm fluid pump which is used as a vacuum pump, a fuel pump or the like.
A conventional diaphragm fluid pump of this kind includes a diaphragm piston assembly assembled within a pump housing to form a pump chamber at one side thereof, which piston assembly comprises a diaphragm member secured in a fluid-tight manner at its outer periphery to the peripheral wall of the pump housing, and an operation rod connected at its one end to the diaphragm member and at its other end to a swingable end of a rocker arm to reciprocate the diaphragm piston assembly in an axial direction so as to produce a positive or negative pressure in the pump chamber. The rocker arm is pivoted on a portion of the pump housing to swing by its engagement with a cam member on a drive shaft. In such an arrangement, the operation rod is applied with forces in its axial and radial directions during operation of the rocker arm, resulting in irregular deformation of the diaphragm member upon reciprocation of the diaphragm piston assembly. This causes a partial stress in the diaphragm member, resulting in fatigue of the diaphragm member in a short period of time. Furthermore, a dish plate is secured to the diaphragm member inside the pump chamber to fasten the operation rod in place, which dish plate swings upon reciprocation of the diaphragm piston assembly. For this reason, it is required to lower the upper dead point of the diaphragm piston a predetermined distance. This reduces the capacity of the pump chamber, resulting in decrease of the volumetric efficiency of the fluid pump.
In order to overcome the above-noted problems, it has been proposed to provide a guide portion in the pump housing for axially slidable support of the operation rod. Such an arrangement of the guide portion is intended to restrict movement of the operation rod in its axial direction to thereby prevent the diaphragm member from experiencing irregular deformation. However, the movement of the operation rod may not be restricted in its axial direction because the operation rod is guided only at one portion thereof. Furthermore, the radial force acting on the operation rod will cause wear and seizure at the guide portion.